Quantum information processing implementations using superconducting qubits have made significant progress towards systems of useful integration complexity in recent years. In the standard conception of gate-based quantum computation, it is important that the qubits are decoupled from one another most of the time, and are selectively coupled during gate operations in a controllable manner. Tunable couplers are a common method of achieving this, and have been demonstrated using superconducting qubits in a number of different implementations. The vast majority of coupler devices and qubit circuitry reported in the literature, and those with the longest-lived qubit coherence times, are designed in coplanar two-dimensional geometries.
In coplanar microwave integrated circuit design, the routing of signal traces and microwave components necessitates a severance of the ground planes due to the two-dimensional constraint of the geometry. The severance of the ground planes allows them to maintain different electrical potentials, and undesired parasitic modes can propagate along the RF signal paths when the ground potentials are unequal. Conventional tunable couplers have used air bridges or wire bonds to suppress the propagation of undesired modes across the coupler. Air bridges are important components used mainly to suppress such undesired modes, but are not available in all 2D circuit fabrication processes. Wire bonds employed as jumpers between ground sections can also be used to suppress such modes, but their effectiveness is limited by their much higher inductance than a continuous ground plane metal interconnect. Different signal propagation times arise at discontinuities such as air bridges and wire bonds, and undesired modes can propagate if the structure is not placed symmetrically inside a circuit layout.